Considerations of energy saving and pollution reduction, especially in the urban environment, lead the automobile manufacturers to equip their models with an automatic start/stop system, such as the system known under the Anglo-Saxon term of “stop and go”.
A typical “stop and go” situation is that of stopping at a red traffic light. When the vehicle stops at traffic lights, the thermal engine is switched off automatically, then, when the traffic lights turn green, the engine is started again by means of the alterno-starter, after the system has detected depression of the clutch pedal by the driver or any other action translating the intention of the driver to re-start his or her vehicle.
A three phase alternator able to function as a starter, that is to say as an electric motor, is described by VALEO ELECTRONIQUE in French patent application FR2745445.
A rectifying bridge on the output of the alternator armature is also used as a bridge to control the phases of the electric motor, power transistors of the arms of the bridge being controlled by sequences of square signals issued by a control unit.
Such full wave control of the “all or nothing” type is well-known to the person skilled in the art and is very simple to implement.
In the latest alterno-starters, a multiphase synchronous rotating electric machine is connected to a reversible AC/DC converter—or inverter—supplied by the main battery in driving mode and by the electric machine in generating mode.
The development of digital technology enables the inverter controlled by modulating the pulse width, commonly called MLI or PWM (“Pulse Width Modulation” in English) inverter to obtain precise control of the electric machine for each particular operating condition.
Examples of MLI commands generated by a microprocessor running programs are described in French patent application FR2895597 under the name of VALEO EQUIPEMENTS ELECTRIQUES MOTEURS.
However the inventive entity was confronted with the problem of limiting the commutation frequency of inverters on electric machines with a large number of poles per phase which rotate quickly.
In fact the software processing which leads to the issue of cyclical reports of MLI commands requires a frequency of the MLI signals considerably higher than an electrical frequency corresponding to the number of revolutions of the machine (typically more than an order of magnitude).
Due to limiting the commutation frequency of the inverters, it is difficult to achieve these ratios if a number of pairs of poles per phase is great (an electric pulsing is equal to the number of revolutions multiplied by the number of pairs of poles per phase).
A solution normally used in these circumstances is to control the phases in full wave mode using a traditional method, that is to say by generating a square waveform at an electric frequency of the machine, based on a predetermined number of nominal revolutions.
Also an operating mode in which the machine is not controlled in an active way and no longer interacts with the electrical network is needed for the machine to function. Such a mode is especially necessary as fallback mode in the event of defect. This mode can also be used when there is no defect, in situations where the machine does not have to provide or consume electrical energy.
The use of a machine with wound excitation enables this mode to be used by cancelling the rotor excitation current and by opening the control switches in order to isolate the phases of the machine from the on-board power system. Another embodiment of such a mode consists in cancelling the rotor excitation current and closing certain control switches in order to short-circuit the phases of the machine.
However it was found by the inventive entity that abrupt transition between this passive mode and a full wave command produced a high peak current.